The polarization of electromagnetic waves (EM) is one of the fundamental properties of optical radiation. Manipulation of the polarization is usually achieved via polarizers. Typical examples of polarizers are right-handed crystals, nematic liquid crystals, anisotropic media, chiral media, optical gratings, and materials possessing the Brewster and Faraday effects. Despite a fairly large selection, there is considerable demand in the industry for the improvement of existing polarizers or the development of new types of polarization converters for the use in modern complex devices. Such polarizers can be chiral metasurfaces which are electrically thin, compact, flexible structures with properties of controlling the polarization of light. Typical chiral metasurfaces consist of chiral subwavelength inclusions arranged into a lattice that can convert the polarization of incident light in transmission or reflection regimes from linear to circular and vice versa  or linear to linear (transverse electric polarized light is converted into transverse magnetic polarized light and vice versa) .
Metasurfaces based on Huygens’ principle have a special place in the wave manipulations due to their high efficiency and simplicity. The Huygens principle is based on the fact that each point of the wavefront acts as a secondary source (electric or magnetic) for the emitting waves. This principle gives a qualitative relation between the radiating field and its source in dipole consideration. Huygens’ metasurface based on chiral inclusions provides more opportunities in the controlling of the light polarization due to the well-defined and tailoring of the induced electric and magnetic dipole moments of chiral polarizable inclusions. As a result, the orthogonal polarizations of linear polarized waves are manipulated independently in Huygens’ chiral metasurface that leads to the near-perfect polarization conversion with the unit transmittance or reflectance, nonsplitting of the resonance, and broad transparency outside of the resonant band .
In this study, we design Huygens’ metasurfaces based on chiral inclusions for the manipulation of polarization state in reflection and transmission regimes for midinfrared (mid-IR) spectral range. By exploiting a comprehensive approach for EM dipole interactions, we analyze how the arrangement of chiral inclusions in a metasurface affects its polarization properties. As chiral inclusions of metasurfaces, we consider smooth metallic helices with axes oriented parallel (horizontally-oriented) and orthogonal (vertically-oriented) relative to the plane of the metasurface. Structural parameters of helices were tailored to achieve balanced excitation of electric and magnetic dipole moments (EM balance) in the metasurface. That leads to the balanced EM response for the excitation by linearly polarized plane waves and polarization insensitivity of the helix-based metasurfaces. In addition, the usage of helices with optimized parameters to the EM balance provides a highly efficient polarization conversion with a broadband low-reflection in the entire spectrum. We determined all components of electric and magnetic dipole moments inducing in horizontally-oriented and vertically-oriented helices to show how the orientation of dipole moments affects the polarization state of reflected and transmitted waves. In accordance with this, we determined the mutual arrangement of helices in the unit cell of metasurfaces for the desired polarization conversion.
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